The invention relates to an assembly of an impedance transformer and a balun transformer, said impedance transformer and said balun transformer being connected in cascade.
The invention also relates to an amplifier and an optical receiver provided with such an assembly.
Such an assembly of an impedance transformer and a balun transformer is known and, inter alia, used in the CATV amplifier module BGD 902 made by Philips. Such CATV amplifier modules are used in coaxial cable networks to maintain the signal level. To this end, these CATV amplifier modules comprise a cascade connection of, respectively, an input stage, an amplifier stage and an output stage. With respect to these CATV amplifier modules, very high distortion requirements are to be met, so that the amplifier stages are generally embodied so as to be symmetrical push-pull amplifiers. Due to this measure, the second-order products formed cancel each other in the output stage. In this respect, it is very important that both amplifier branches are symmetrically driven and loaded. An additional advantage obtained by symmetrically driving and loading the amplifier branches is that relatively few third-order distortion products are introduced into the output signal. In the known CATV amplifier module, the output stage consists of an assembly of an impedance transformer and a balance-unbalance transformer, also known as balun transformer. In the known CATV amplifier module, the impedance transformer ensures that a desired impedance transformation between the amplifier stage and the output of the amplifier module is obtained, so that an optimum power transfer between the output of the amplifier module and the network can be achieved. The balun transformer matches the symmetrical amplifier outputs are to an asymmetric output.
The construction of the known assembly of an impedance transformer and a balun transformer is relatively complicated and expensive. In addition, the known assembly causes relatively high signal losses.
It is an object of the invention to provide an assembly of the type mentioned in the opening paragraph, the construction of which is relatively simple and inexpensive, and which assembly causes relatively low signal losses.
To achieve this, the assembly in accordance with the invention is characterized in that the impedance transformer and the balun transformer are mounted on a single core of magnetic material.
In the known assembly, the impedance transformer and the balun transformer both have their own core. By combining the impedance transformer and the balun transformer on a single core, an assembly is obtained which is functionally equal to the known assembly but the construction of which is simpler and smaller. It has further been found that the signal losses have decreased with respect to the known assembly.
A first embodiment of the assembly in accordance with the invention is characterized in that the balun transformer comprises a transmission line transformer. By virtue of the fact that the primary and the secondary winding in a transmission line transformer exhibit a uniform capacitive coupling, the use of such a transmission line transformer results in a balun transformer exhibiting a relatively good symmetry over a relatively large frequency range.
A second embodiment of the assembly in accordance with the invention is characterized in that the impedance transformer comprises an autotransformer. As the autotransformer has only one winding, the impedance transformer is relatively simple and inexpensive.
A third embodiment of the assembly in accordance with the invention is characterized in that the autotransformer comprises a series connection of, respectively, first, second, third and fourth coils, which series connection is arranged between a first and a second terminal, the series connection comprising a first branching point between the first and the second coil, and the series connection comprising a first ground point between the second and the third coil, the series connection comprising a second branching point between the third and the fourth coil, and the transmission line transformer comprising a primary and a secondary coil, the primary coil being arranged between the first branching point and a third terminal, and the secondary coil being arranged between the second branching point and a second ground point, a first number of turns of the primary coil being equal to a second number of turns of the second coil, and a third number of turns of the third coil being equal to a fourth number of turns of the secondary coil, the first number of turns being equal to the fourth number of turns. Due to the fact that the transmission line transformer and the autotransformer are mounted on the same core, a magnetic coupling is formed between the coils of these two transformers. This magnetic coupling is such that if the number of turns of the secondary coil of the transmission line transformer is unequal to the number of turns of the third coil of the autotransformer, a large and undesirable compensating current starts to flow between the parallel-arranged coils. If, on the other hand, the number of turns of the secondary coil and the third coil are equal, the voltages carried by both coils are in-phase and of the same amplitude, so that the compensating current does not occur anymore.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.